For the control technique of a conventional type permanent-magnet type synchronous motor provided with a polar position sensor, there is a control device disclosed in a patent document 1. The control device utilizes positional information from the polar position sensor for a limiting value of a rotational phase directed value and prevents the loss of synchronism by the rapid change of a load and others.
Besides, in a patent document 2, for a control device without a position sensor of a synchronous motor, technique for calculating axial displacement based upon a current sensed value and a voltage directed value to a power converter and adjusting an output frequency of the converter based upon the axial displacement is disclosed.
[Patent Document 1]
JP-A 324881/2000 (Abstract, Paragraph 0011 and others)
[Patent Document 2]
JP-A 251889/2001 (Claim 9, Paragraph 0105 and others)
In the prior art of the patent document 1, the axial displacement is operated, however, in a torque control system, the axial displacement is used only for the limiting value of the rotational phase directed value acquired by integrating a frequency directed value given from a host and high-precision torque control cannot be expected.
Besides, in the prior art of the patent document 2, in case it is difficult to give sufficient high-speed responsibility to a frequency arithmetic unit for operating a frequency based upon axial displacement, torque control precision particularly in acceleration/deceleration is not enough.